The most basic optical property parameters of an optical glass are refractive index and dispersion. Refraction is generated due to the change in the propagation direction of light in a medium relative to in air (or vacuum), and dispersion is generated due to a decrease of refractive index of a same medium with decreasing of the frequency of an incident light. In an optical glass, Abbe number is associated with dispersion, and the higher the dispersion, the lower the Abbe number. Therefore, a high refractive index of an optical glass indicates that the propagation direction of light is changed greatly by the optical glass. High dispersion of an optical glass indicates that the optical glass has a strong dispersion effect on light and a good spectral separation effect; and low dispersion of an optical glass indicates that the optical glass has a weak dispersion effect on light and a good imaging effect. When a spectroscopic study on light is required, it is desirable to obtain a lens with high dispersion so as to separate lights with different wavelengths more clearly. In modern photo-optical systems, it is always desirable to obtain a lens having low dispersion and a good imaging effect. Therefore, no matter in what kind of invention area, one optical glass is generally difficult to meet specific requirements, and usually a number of optical glasses with different dispersions are used. In the cases where dispersion correction can be performed, optical glasses with relatively high refractive indexes are often used, making an optical glass with high refractive index more and more important in the field of optical glass. In particular, an optical glass having a refractive index of 1.80 or more and dispersion in a middle range (that is, Abbe number being in the range of 30 to 40) are usually used.
In recent years, with the development of the optoelectronic industry, optical equipments are demanded to meet the requirements of miniaturization, light weight and high performance. In order to reduce the number of lenses constituting the optical system in an optical equipment, aspheric lenses are increasingly used in optical designs. Precision press molding is presently wildly used for manufacturing aspheric lenses. The so-called precision press molding means that a glass preform is molded using a high-precision mold with a predetermined product shape under certain temperature and pressure conditions to obtain glass products having the product shape and an optical functional surface. For aspheric lenses manufactured by using the precision press molding, it is not needed to preform further grinding and polishing, and thus the cost is reduced and the production efficiency is increased.
When the precision press molding is performed, in order to transfer a high-precision mold surface to a glass molded article, it is necessary to conduct pressure-molding on a glass preform at a high temperature. At this time, the pressing mold is exposed to a high temperature environment and subjected to a high pressure, and even if the pressing mold is in a protective atmosphere, the molding surface of the pressing mold is still easy to be oxidized and eroded. In precision press molding methods, high-precision molds are the main source of cost, and when a mold cannot be used a certain number of times, low cost and high yield will not be achieved. In order to extend the service life of the mold and reduce damages to the mold caused by the high temperature environment, it is necessary to reduce the temperature of the press molding as much as possible. Therefore, the transition temperature Tg of the glass material used is required to be as low as possible.